In E-UTRAN Orthogonal Frequency Division Multiple Access (OFDMA) technology is used in the downlink. OFDM is a modulation scheme in which the data to be transmitted is split into several sub-streams, where each sub-stream is modulated on a separate sub-carrier. Hence in OFDMA based systems, the available bandwidth is sub divided into several resource blocks or units as defined, for example, in 3GPP TR 25.814: “Physical Layer Aspects for Evolved UTRA”. According to this document, a resource block is defined in both time and frequency. According to the current assumptions, a resource block size is 180 KHz and 0.5 ms in frequency and time domains, respectively. The overall uplink and downlink transmission bandwidth is as large as 20 MHz.
Downlink Measurements in E-UTRAN
In E-UTRAN the user equipment (UE) is required to perform different types of measurements in order to facilitate many radio resource management (RRM) related tasks such as scheduling, handover, admission control, congestion control etc. Some typical downlink measurements carried out by the UE include channel quality indicator (CQI), carrier received signal strength indicator (carrier RSSI), reference symbol received power (RSRP) etc. Some of these measurements are done on the reference symbols, which are transmitted at least once every 6th sub-carrier in the frequency domain. For instance RSRP and CQI are measured over the reference symbols. Other common channels sent on the downlink in E-UTRAN include synchronization channel (SCH) and broadcast channel (BCH).
Some of these measurements especially RSRP, which is based on long term averaging, is used by the network for coverage triggered handover. In other words RSRP could provide information related to the cell coverage.
Packet Oriented Transmission in E-UTRAN
The E-UTRAN is a packet oriented system, where all type of uplink and downlink transmission including data and signalling takes place via shared channel. The network has the full control over the radio and network resources or the so-called scheduling grants (e.g. resource blocks, modulation, coding, sub-frames etc), which are assigned to the UE on demand basis. However, it might be possible, partly or fully, to pre-assign limited amount of resources to the UE for certain type of critical periodic measurement reports.
RACH Transmission in E-UTRAN
Random access in E-UTRAN is based on a two-step procedure. In the first step the UE transmits a randomly selected signature to the network. In the subsequent procedure the network responds to the UE with an uplink scheduling grant which is used by the UE to transmit further details related to the connection request.
The transmitted sequence is randomly chosen out of an available pool of 64 unique sequences. If the UE does not receive an uplink scheduling grant within a certain time it randomly selects a new signature sequence and performs a new random access (RA) attempt.
The time-frequency resources where random access may be performed are announced via system information. One random access opportunity (or resource) is 1.08 MHz wide (6 resource blocks, each comprised of 180 kHz in the frequency domain) and lasts for 1 ms. Multiple RA opportunities may be spread out over frequency. It is up to the network whether to schedule other data in a RA slot or not. The network thus also controls whether RA transmission is orthogonal to shared data transmission or not.
Out-of-Coverage Concept
An explicit out of coverage concept is not specified. However, similar concept called out of sync is used in the WCDMA. In WCDMA the downlink power control is mandatory. This means the base station adjusts its downlink transmitted power in response to the power control commands (TPC) sent by the UE. In case of out of sync situation there is a risk that excessive power up commands may saturate the base station downlink transmitted power. Thus the main purpose of out of sync in WCDMA is to protect the base station from transmitting unnecessary high power. In other words when out of sync is reported the base station disables the UE connection or at least simply ignore the received TPC commands from the UE. The general concept of out of sync concept is described below.
The UE monitors downlink channel quality on a suitable measurement signal (e.g. reference or pilot signals). If the estimated downlink quality remains below an acceptable quality limit (Qout) over time period (Tout) then the UE reports out-of-coverage to its higher layers (e.g. layer-3 or RRC) through an out-of-coverage primitive. Subsequently UE higher layers indicate to the RRC in the network (e.g. RNC) that the UE physical layer has detected out-of-coverage. The network then takes an appropriate measure such as change downlink power offsets, handover, congestion control etc.
Since the downlink radio condition improves, hence the UE also needs to monitor downlink channel quality when in the state of out-of-coverage. In this situation if the UE detects that the estimated downlink quality has become greater than another threshold (Qin) over time period Tin, then the UE reports in-coverage to its higher layers through primitive. Subsequently the UE higher layers inform the network that its physical layer has detected in-coverage.
Out of Sync Concept in WCDMA
As stated above no explicit out-of-coverage concept is specified. However, similar concept called out-of-sync reporting procedure is used in WCDMA to protect the base station transmitted from transmitting unnecessary high power.
In WCDMA downlink out-of-sync is specified in the specification document 3GPP 25.214 “Physical Layer Procedures” and is briefly described as follows:
The UE estimates downlink channel quality, which is expressed as transmit power control (TPC) command error rate. If the downlink quality is below specified level (Qout) over Tout the UE reports out-of-sync. Typically out of sync is reported to the network by the UE if the downlink measured quality in terms of TPC command error rate exceeds 30% over 160 ms measurement interval.Since TPC commands are sent on DPCCH or F-DPCH, therefore the out of sync criteria is based on the dedicated channel, i.e. on UE specific channel. An overview of the out of sync procedure in WCDMA is depicted in FIG. 1.Scenarios for Out of Coverage in E-UTRAN
In this section we described some important scenarios or situations where explicit coverage loss indication is needed by the network. Some examples of such scenarios are:                E-UTRAN coverage border        Poor coverage at the junction of irregular cell sizesCoverage Border Scenario:        
Limited E-UTRAN coverage in some geographical vicinity may result in coverage boundaries. This may easily lead to a situation where a UE loses coverage when moving out of the E-UTRAN coverage borders. An explicit coverage loss indication in this scenario would make the network aware of the UE coverage status more clearly. This could help the network to improve the coverage, if possible, by employing suitable radio resource management techniques. On the other hand, a consistent coverage loss as depicted by the coverage loss criteria could allow the network to drop such a UE well in time thereby saving network resources.
Poor Coverage Areas:
E-UTRAN like any other cellular network is expected to offer ubiquitous coverage in all types of locations. However, there are at least some inevitable coverage spots, where good cell planning is very hard to realize in practice. These areas could be found at the intersection of very irregular cell sizes e.g. cells encompassing hilly and skyscrapers within a congested metropolitan region. Consistently ensuring good coverage in such awkward coverage spots could drain enormous network radio resources. However, an efficient radio resource management mechanism could react and improve the coverage on demand basis by allocating more resources etc. The realization of such a mechanism requires explicit UE feedback when its coverage falls below the desired level.
Limitation of Implicit Coverage Loss Indication
The UE reported measurements such as RSRP in E-UTRAN may implicitly depict the coverage status of the UE. However, any measurement including RSRP is limited to a certain minimum reporting value, e.g. up to −140 dBm. However, in such out of coverage scenarios the measured quantity (e.g. RSRP) is likely to be out of the reporting range, i.e. much lower than the minimum reportable value. In case the reported range is further extended (e.g. below −140 dBm), the measured results at lower ranges would obviously incorporate very large measurement uncertainties. This is because at low measured quantity the uncertainty becomes larger rendering the reported value highly unreliable. Hence in these circumstances, solely based on the UE measurement reports, the network may not correctly infer that the UE is out of coverage or not.
Coverage Indication via Dedicated Connection
In out of coverage or out of sync scenario the downlink connection (from base station to UE) becomes unreliable. In WCDMA, where a dedicated connection is maintained, the UE is still able to send out of sync indication on the uplink despite the downlink remains unreachable. In E-UTRAN only shared channel is used for uplink and downlink transmission, where resources are allocated by the network on demand basis. It is thus unlikely that UE in E-UTRAN will be able to correctly receive any scheduling grant or resource allocation from the network in out of coverage situation. Thus the current WCDMA approach is less likely to work in E-UTRAN.
Coverage Loss Criteria based on Dedicated Reference Signals
The existing out-of-sync procedure in UTRAN takes into account only the DPCCH or F-DPCH when estimating the downlink channel quality. As a consequence the downlink channel quality estimate is done on dedicated pilot bits and/or TPC commands.
However, it has been evaluated that downlink channel quality estimate based on both dedicated and common pilots leads to better out-of-sync detection by the UE. The possible reason is that dedicated reference signals or pilots used in WCDMA are power controlled that does not provide the actual coverage status of the UE. The aspect of using the common reference signals to detect out-of-sync or out-of-coverage is not used in the existing system.